Permanent magnet synchronous motors (PMSMs) are utilized in various applications because they have generally favorable efficiency characteristics relative to other types of motors. Typically, PMSMs have three separate electrical windings within the stator that are powered by an alternating current (AC) voltage source. The shaft torque of the motor and the power conversion efficiency depend upon both the magnitude and the phase angle of the AC voltage.
In certain applications, such as electric vehicles and hybrid electric vehicles, electrical power is available from a direct current (DC) voltage source such as a battery. Therefore, inverters are utilized to convert the DC voltage into the three-phase AC voltage. Inverters contain a discrete number of switching devices and are therefore capable of supplying only a discrete number of voltage levels at each of the three motor terminals. For a 2-level inverter, at any moment in time, the switching devices are set to electrically connect each of the three AC terminals to either the positive or the negative DC terminal. Thus, eight switching states are available. Two of these switching states, in which all three terminals are connected to the same DC terminal, are called zero states. In the remaining six states, one AC terminal is connected to one of the DC bus terminals and the other two AC terminals are connected to the opposite DC bus terminal.
In one basic control method, called six-step, the inverter cycles through the six non-zero states once per electrical cycle of the rotor, producing AC voltage and current in the windings. An electrical rotor cycle is defined relative to motor poles and does not necessarily correspond to a complete revolution. The amplitude of the AC voltage is dictated by the DC voltage. The torque is dictated by the DC voltage, the rotor speed, and the phase difference between these quasi-sinusoidal AC voltage signals and the rotor position. A controller issues commands, called gate control commands, indicating to the inverter when to switch to the next state in the sequence. Accurate torque control requires precise timing of these inverter gate control commands.